In 1968 Pope Paul VI's promulgation of Humanae vitae reaffirming the condemnation of birth control was profoundly controversial and has had a powerful impact on Catholic moral theology ever since. It provides an important case study of the varied conflicting pressures on a question of change in the Church's moral teaching. In my presentation first I sketch the Birth Control Commission's deliberations and its final report calling for an end to the Church's condemnation. I sketch how Pope Paul decided to go against the Commission's view despite the Vatican Council's recent emphasis on the need for "collegiality" within the Church. Humanae vitae triggered sharp debates about sexual ethics, the status of papal encyclicals and papal authority, and the relevance of natural law moral reasoning. Second I examine how in 2015 Pope Francis published Laudato 'si the Church's historic first encyclical focusing on mounting ecological concerns and especially on climate change. While Francis repeats the ecological understanding that "Everything is connected," he suggests—wrongly--that humanity's surging population growth is little connected to our mounting environmental problems. In 1968 human population stood at 3.5 billion. Today it stands at 8.2 billion. Fourth, I suggest that Catholic liberals are wrong to drop interest in the natural law tradition and that Catholic conservatives are wrong to try to sustain the church's condemnation of birth control. I sketch how rising ecological concerns helpfully remind us of the prominent stress on the doctrine of creation and of the natural law in Thomas Aquinas's writings. Thomas holds that the natural law can change due to changes in circumstances. Accordingly, I sketch ecologically-informed natural law reasons why, given the significance of climate disruption and other ecological concerns, the use of birth control should cease to be judged as an intrinsically evil act.